1. Field of the Invention
The present invention relates generally to solid investment molding by the lost wax process. Particularly, the present invention relates specifically to structures used in the formation of solid investment molds and methods of use of those structures.
2. Description of the Prior Art
The lost wax casting process involves the formation of a pattern of the desired object to be cast. The pattern is customarily formed of wax or plastic having the desired burnout characteristics. The wax pattern or wax positive, to which sprues of the same material as the pattern have been attached, is then embedded in a mixture of refractory investment materials such as Plaster of Paris. The resulting invested pattern is then subjected to intense heat in order to drive out moisture from the investment material and to completely eliminate the wax or plastic used for the pattern and sprue. The burnout procedure results in the formation of a mold cavity in the investment mass. Molten metal is then introduced into the mold cavity by gravity feed, vacuum-assisted gravity feed or centrifugal casting methods and the resultant cast is recovered by destruction of the investment mass.
A structure is required in the solid investment molding process to contain the initial refractory investment materials that are poured around the wax pattern to form a reverse mold of that pattern. Prior art structures used to initially form solid investment molds for use in the lost wax process include a solid-walled metal flask, a metal perforated flask with a casting chamber, a solid-walled metal flask with a wax flask liner, and a solid-walled metal flask with an artisan-made chicken wire form.
The solid-walled metal flask gives rigidity to an investment mold during the metal casting process. The non-porous structure of the solid-walled metal flask does not allow moisture and impurities to be easily burned off during the heating process. It also makes it difficult to remove the investment mold from the solid-walled metal flask when the entire process is complete.
The perforated stainless steel flask for use with a casting chamber was developed to more easily allow moisture and impurities to be burned off during the heating process. U.S. Pat. No. 5,257,658 (1993, Perera), discloses a perforated stainless steel casting flask as it is used in vacuum casting within a casting chamber. During the initial investment material pouring process, the perforated flask requires an external sheath to keep the investment from leaking out until the molding investment material sets up. The perforated stainless steel casting flask allows gas to be evacuated through the holes within the flask during the final casting where the heated metal liquid such as bronze is poured into the flask. The perforated flask, like the solid walled flask, also requires pressure or force to be applied to remove the investment mold from the flask.
The wax flask liner was developed as an alternative to the perforated metal flask. The wax flask liner is a wax mesh sheet, sold under the trademark of “Wax Web,” that is placed up against the inside surface of the solid-walled, cylindrical flask during the assembly of the investment molding structure. A wax sprue and a wax pattern are placed inside the metal flask cavity. The liquid investment material, which is generally Plaster of Paris, is then poured into the metal flask surrounding the wax sprue and the wax pattern. Once the investment material has set up at room temperature, the solid-walled metal flask containing the wax pattern, the investment material, and the wax mesh sheet is heated in a kiln. The heating process removes water and other impurities from the investment material, cures the investment material, and burns out the wax pattern and the wax mesh sheet. The porous cavities left after the wax mesh sheet is burned away allows moisture and other impurities to escape more easily from the curing investment material during this kiln-heating process, which is also called the burnout process.
After the investment material has cured and the wax burned out, a molten metal such as bronze is poured into the mold cavity and allowed to cool to solidify the molten metal. Because the metal flask is a single piece, cylindrical tube, the investment mold is difficult to remove. To remove the investment mold from the metal flask, pressure or force must be applied to the investment mold to separate it from the metal flask.
For relatively large investment molds, chicken wire has been used by artisans in an investment molding system to create initial investment mold structures of varying size and shape. A diagram of solid investment mold making that shows an example of a chicken wire structure is disclosed in Sculpture Journal, Vol. 6, No. 4 (June 2002), p. 24. Chicken wire is flexible and can be bent to form an unusually sized investment molding structure. Chicken wire forms, due to a relatively large ratio of opening size to solid wire surface area that is larger than the wax liner or the perforated flask, have allowed for better moisture removal, cleaner more complete burnout, and good gas flow during the introduction of the molten metal than the other prior art structures hereinbefore disclosed. With the added reinforcement provided by the chicken wire after the plaster as set up, there is generally no longer a need for an external containment vessel to remain with the mold through the burn out and metal pouring steps.
However, such a mold is prone to cracking and may need to be handled carefully and given additional reinforcement, often by packing sand around the mold before pouring the molten metal. Other drawbacks of the chicken wire system are that it has only tensile strength and little, if any, compression strength. It also does not lend itself to vacuum assist pouring. It is very time consuming to cut and form the chicken wire to the desire shape and protective gloves and care must be used to avoid injury from the chicken wire.
In addition, chicken wire is usually galvanized with a metal that prevents oxidation. While galvanic coating is useful to prevent corrosion of the chicken wire when exposed to the elements, it is detrimental when used as an investment molding structure. The galvanization creates undesired gases at high temperatures that can potentially cause contamination of the investment mold. Further, the burn out process causes the chicken wire structure to lose its galvanized surface, thereby allowing the chicken wire form to oxidize. Oxidation and other physical changes in chicken wire during the burn out process render the chicken wire form entirely altered and not recommended for re-use. Although chicken wire forms can be easily pulled away from the investment mold, the chicken wire form becomes disfigured from this extraction process and cannot be easily re-formed for re-use. Thus, chicken wire forms must be newly created each time a new investment mold is made.
Therefore, what is needed is an investment molding structure that is reusable. What is also needed is an investment molding structure that allows for quick moisture removal, clean and complete wax burn out, and good gas flow. What is further needed is an investment molding structure that has both tensile strength and compression strength for use in both hand pouring and vacuum assisted pouring. What is still further needed is an investment molding structure that is flexible and may be easily formed into varying shapes and sizes. What is yet further needed is an investment molding structure that does not require the use of protective gloves to prevent injury during investment structure assembly. What is also needed is an investment molding structure that is collapsible for easy storage. What is even further needed is an investment molding structure that can be more easily removed from the waterproof sleeve without the need for additional pressure or force. What is needed is an investment molding structure that does not need additional external reinforcement for receiving the molten metal after removal from the containment structure.